In gas turbines, noises develop on account of the rotational movement of the rotor and the blades in conjunction with fluidic effects and are emitted to the outside via the turbine casing, for example. The air intake region of the gas turbine emerges as a particularly powerful source of noise because provision is made there for devices such as the intake manifold (see U.S. Pat. No. 7,246,480 B2) which introduces large air flows into the intake region of the compressor of the gas turbine, and which are equipped with large-area walls, usually constructed from metal sheets, which radiate the internally developed sound easily and effectively to the outside.
FIG. 1 shows an exemplary air intake region of a gas turbine 10 which drives a generator 11 (shown only in outline). The gas turbine 10 has a machine axis 16 around which the rotor (not shown) of a compressor rotates. The compressor has an air intake 12 in the form of an annular intake nozzle (“bell mouth”) into which air 15—which may be additionally processed beforehand, e.g. filtered, or provided with an atomized spray—is directed by means of an intake manifold 13.
The intake manifold 13 is delimited on the outside by means of large-area walls 14 which are customarily constructed from metal sheets and are provided with a frame structure and possibly with stiffening struts. The walls 14 can especially be of a double-walled design. The large-area metal sheets in this case act as loudspeaker membranes and emit the sound which propagates inside the air intake region to the outside, largely without being damped.
An active intake silencer for an air intake duct of an internal combustion engine is known from printed publication EP1717433A2, having sensors which are connected to a control unit which actuates a converter. Provision is made for a special heat-resistant and moisture-resistant membrane which is acoustically coupled with the intake air flow in the air intake duct, wherein the surface of the membrane facing the intake air flow is made to flexurally oscillate by means of the converter for producing structure-borne sound which is attuned to the intake air sound. Such an arrangement is only suitable for small flow cross sections and requires a special, additionally installed membrane.
GB2204916A proposes a combination of a passive silencer and an active silencer, operating with loudspeakers, for the intake or exhaust of a gas turbine for noise reduction. The high equipment cost and the fluidic influence of the arrangement are disadvantageous in this case.
For an aircraft power plant, printed publication U.S. Pat. No. 5,423,658 proposes an active silencing system in which the resonance frequency of the sound-radiating structure is tuned so that the sound can be optimally quenched over a large frequency range. For tuning the resonance frequency, provision is made for a multiplicity of actuators which tune the resonance frequency of the metal sheets of the casing in the desired manner. The metal sheets are then made to oscillate by means of attached oscillation transducers in order to create the necessary compensating countering sound. A disadvantage is the comparatively high cost of equipment and control engineering as a result of the actuators and oscillation transducers.
An actively controlled acoustic wall, which is provided for the casings of gas turbines, is known from printed publication U.S. Pat. No. 5,702,230 A. The wall consists of a rear wall, which consists of a planar matrix of individually controllable elements which are arranged side-by-side and in each case comprise a transducer in a honeycomb-like chamber, and an attached front wall. Such an acoustic wall, on account of its small-sized, complicated construction, is extremely costly in production, especially if it involves larger areas.